Determination of cysteine in human plasma by high-performance liquid chromatography and ultraviolet detection after pre-column derivatization with 2-chloro-1-methylpyridinium iodide

This paper describes the development and validation of a reversed-phase HPLC method with isocratic conditions for the analysis of total cysteine in human plasma. The essential steps in the assay include derivatization of cysteine via thiol group with 2-chloro-1-methylpyridinium iodide (CMPI), separation of the formed cysteine S-pyridinium derivative from endogenous thiol derivatives, usually present in biological samples, by ion-pair reversed-phase HPLC, and ultraviolet detection. Free cystine and protein-bound cysteine are converted to their reduced counterpart by the use of 2-mercaptoethanol. The standard curve is linear over the concentration range from 20 to 300 nmol ml−1. The recoveries are from 93.9 to 105.9% and imprecision was at most 2.5%. Optimal conditions are determined for all steps of the analytical procedure.